A major source of environmental pollution is the production of energy. While research into alternative, cleaner sources of energy has grown, the vast majority of the energy produced in the world is still obtained from fossil fuels such as coal, natural gas and oil. In fact, in 2005, 75% of the world's energy was obtained from fossil fuels (Environmental Literacy Council). Of these fossil fuels, coal provides 27% of the world's energy and 41% of the world's electricity. Thus, there is also increased interest in making current energy producing processes more environmentally friendly (i.e., cleaner).
Coal is an abundant source of energy. Coal reserves exist in almost every country in the world. Of these reserves, about 70 countries are considered to have recoverable reserves (World Coal Association). While coal is abundant, the burning of coal results in significant pollutants being released into the air. In fact, the burning of coal is a leading cause of smog, acid rain, global warning, and toxins in the air (Union of Concerned Scientists). In an average year, a single, typical coal plant generates 3.7 million tons of carbon dioxide (CO2), 10,000 tons of sulfur dioxide (SO2), 10,200 tons of nitric oxide (NOx), 720 tons of carbon monoxide (CO), 220 tons of volatile organic compounds, 225 pounds of arsenic and many other toxic metals, including mercury.
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO2). Free radicals of nitrogen (N2) and oxygen (O2) combine chemically primarily to form NO at high combustion temperatures. This thermal NOx tends to form even when nitrogen is removed from the fuel. Combustion modifications, which decrease the formation of thermal NOx, generally are limited by the generation of objectionable byproducts.
Mobile and stationary combustion equipment are concentrated sources of NOx emissions. When discharged to the air, emissions of NO oxidize to form NO2, which tends to accumulate excessively in many urban atmospheres. In sunlight, the NO2 reacts with volatile organic compounds to form ground level ozone, eye irritants and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions to low levels. Despite advancements in fuel and combustion technology, ground level ozone concentrations still exceed federal guidelines in many urban regions. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement stringent NOx emissions regulations. Such regulations will require low NOx emissions levels that are attained only by exhaust after treatment.
Exhaust-after-treatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods are known in the art and involve selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Such after-treatment methods typically require some type of reactant such as ammonia or other nitrogenous agent for removal of NOx emissions.
SCR is performed typically between the boiler and air (pre) heater and, though effective in removing nitrogen oxides, represents a major retrofit for coal-fired power plants. SCR commonly requires a large catalytic surface and capital expenditure for ductwork, catalyst housing, and controls. Expensive catalysts must be periodically replaced, adding to ongoing operational costs.
Combustion exhaust containing excess O2 generally requires chemical reductant(s) for NOx removal. Commercial SCR systems primarily use ammonia (NH3) or urea (CH4N2O) as the reductant. Chemical reactions on a solid catalyst surface convert NOx to N2. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Excess NH3 needed to achieve low NO levels tends to result in NH3 breakthrough as a byproduct emission.
Large catalyst volumes are normally needed to maintain low levels of NOx and inhibit NH3 breakthrough. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants. Even under normal operating conditions, the SCR method requires a uniform distribution of NH3 relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed non-uniformly, so low levels of both NOx and NH3 breakthrough may be achieved only by controlling the distribution of injected NH3 or mixing the exhaust to a uniform NOx level.
SCR catalysts can have other catalytic effects that can undesirably alter flue gas chemistry for mercury capture. Sulfur dioxide (SO2 can be catalytically oxidized to sulfur trioxide, SO3 which is undesirable because it can cause problems with the operation of the boiler or the operation of air pollution control technologies, including the following: interferes with mercury capture on fly ash or with activated carbon sorbents downstream of the SCR; reacts with excess ammonia in the air preheater to form solid deposits that interfere with flue gas flow; forms an ultrafine sulfuric acid aerosol, which is emitted out the stack.
Although SCR is capable of meeting regulatory NOx reduction limits, additional NOx removal prior to the SCR is desirable to reduce the amount of reagent ammonia introduced within the SCR, extend catalyst life and potentially reduce the catalyst surface area and activity required to achieve the final NOx control level. For systems without SCR installed, a NOx trim technology, such as SNCR, combined with retrofit combustion controls, such as low NOx burners and staged combustion, can be combined to achieve regulatory compliance.
SNCR is a retrofit NOx control technology in which ammonia or urea is injected post-combustion in a narrow temperature range of the flue path. SNCR can optimally remove up to 20 to 40% of NOR. It is normally applied as a NOx trim method, often in combination with other NOx control methods. It can be difficult to optimize for all combustion conditions and plant load. The success of SNCR for any plant is highly dependent on the degree of mixing and distribution that is possible in a limited temperature zone. Additionally, there can be maintenance problems with SNCR systems due to injection lance pluggage and failure.
Other techniques have been employed to control NOx emissions. Boiler design and burner configuration, for example, can have a major influence on NOx emission levels. Physically larger furnaces (for a given energy input) can have low furnace heat release rates which lead to decreased levels of NOx. The use of air-staged burners and over-fire air, both of which discourage the oxidation of nitrogen by the existence of sub-stoichiometric conditions in the primary combustion zone, can also lead to lower levels of NOx. Over-fire air employs the same strategy as air-staging in which the oxidation of nitrogen is discouraged by the existence of sub-stoichiometric conditions in the primary combustion zone.
Another major contaminant of coal combustion is mercury. Mercury enters the furnace associated with the coal, it is volatilized upon combustion. Once volatilized, mercury tends not to stay with the ash, but rather becomes a component of the flue gases. If remediation is not undertaken, the mercury tends to escape from the coal burning facility, leading to severe environmental problems. Some mercury today is captured by pollution control machinery, for example in wet scrubbers and particulate control devices such as electrostatic precipitators and baghouses. However, most mercury is not captured and is therefore released through the exhaust stack.
In addition to wet scrubbers and particulate control devices that tend to remove mercury partially from the flue gases of coal combustion, other methods of control have included the use of activated carbon systems. Use of such systems tends to be associated with high treatment costs and elevated capital costs. Further, the use of activated carbon systems leads to carbon contamination of the fly ash collected in exhaust air treatments such as the bag house and electrostatic precipitators.
There is a need for an additive and treatment process to reduce emissions of target contaminants, such as nitrogen oxides and mercury.